Method and apparatus of terminating a high voltage solid state device

ABSTRACT

Termination of a high voltage device is achieved by a plurality of discrete deposits of charge that are deposited in varying volumes and/or spacing laterally along a termination region. The manner in which the volumes and/or spacing varies also varies between different layers of a multiple layer device. In a preferred embodiment, the variations are such that the field strength is substantially constant along any horizontal or vertical cross section of the termination region.

TECHNICAL FIELD

[0001] This invention relates to electronics, and more specifically, toan improved technique of terminating a solid state device. The inventionhas particular application in high voltage termination of chargecompensated devices.

BACKGROUND OF THE INVENTION

[0002]FIG. 1 shows a side cross sectional view of an exemplary prior artVDMOS device. As indicated therein, P doped regions 109 are repetitivealong a top surface 114 and are typically kept at approximately groundvoltage during operation of the device. The gate 111 may be operated ata conventional value of, for example, 15 volts.

[0003] At the lower surface of the device is the 600 volt terminal 113.As a result of the structure of the device, that voltage appears atpoint 103 since point 103 is not electrically isolated from the bottomterminal 113 of the device having the 600 volts. In similar devices, thevoltage may rise to 1000 V or more.

[0004] A region 107 is denoted T for termination, and must drop the 600volts across the width of the region. In practical devices, T 107 may beon the order of 50 microns.

[0005] A top view of the arrangement of FIG. 1 is shown in FIG. 2. Theborder region 107 is the termination region, which must include sometype of structure for dropping the 600 volts across only 50 micrometers.Section 105 represents the active region of the device.

[0006]FIG. 3 shows a typical prior art structure for providingtermination of such a high voltage device. A set of floating guard rings302 is used to control the electric field distribution around the deviceperiphery. The number of rings in the structure depends on the voltagerating of the device. For example, 8 rings are used for a 1,000 voltdevice. A three dimensional computer model enables the optimum ringspacing to be determined so that each ring experiences a similar fieldintensity as the structure approaches avalanche breakdown. The rings arepassivated with polydox, which acts as an electrostatic screen andprevents external ionic charges inverting the lightly doped N- interfaceto form P- channels between the rings. The polydox is coated with layersof silicon nitride and phosphorous doped oxide, as shown.

[0007] The surface area of the termination region of the devicerepresents an source of added cost to the device. Specifically, thetermination region is a substantial sized lateral width that must wrapentirely around the periphery of the device. This increases the cost ofthe device, and over the large number of chips per wafer, becomes asignificant source of wasted cost and space.

[0008] In view of the foregoing, there exists a need in the art for animproved technique of terminating high voltage semiconductor deviceswithout utilizing the relatively large amount of surface area.

[0009] There also exists a need for a technique of fabricating atermination structure that is easily manufactured, and does not addsignificant costs to the device manufacturing procedure.

SUMMARY OF THE INVENTION

[0010] The above and other problems of the prior art are overcome inaccordance with the present invention. A multiple layer solid statedevice is constructed wherein each layer includes a varying chargeprofile extending laterally through the termination region, from theedge of the active (drift) region extending laterally towards the edgeof the crystal.

[0011] The charge profile, as defined herein, represents the density ofdeposited charge as a specified cross section is traversed. In apreferred embodiment, the charge profile is different in differentlayers, so that each layer of the multiple layer device includesdecreasing charge density as the termination layer is traversedlaterally. Moreover, a decreasing charge profile is also exhibited as avertical cross section is traversed upwardly towards the source regionof the device. In a preferred embodiment, the charge profile decreasessubstantially linearly along any cross section, lateral or vertical,resulting in a substantially uniform value of electric field strength.

[0012] A preferred method of making the device comprises depositingvolumes of charge along each layer in a multiple layer device, in thetermination region. The volume of charge in each deposit (i.e., dot) orthe spacing between the deposits may be varied, with such variationbeing different at different layers. This causes the field strength toremain substantially constant along any horizontal or vertical crosssection.

[0013] A further understanding will be gained by reference to theaccompanying drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross sectional view of a semiconductor device,depicting the need for termination;

[0015]FIG. 2 is a top view of the arrangement of FIG. 1;

[0016]FIG. 3 shows a prior art termination technique utilizing severalfloating P rings;

[0017]FIG. 4 is a conceptual representation of charge profiles in asemiconductor device, the charge profiles varying in accordance with thepresent invention;

[0018]FIG. 5 is an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019]FIG. 4 shows a conceptual diagram of a cross section through amulti layer device fabricated in accordance with the teachings of thepresent invention. While layers 403, 405, and 407 are shown, furtherlayers are possible. Region 421 represents the active region of thedevice, and region 422 represents the termination region of the device.That active region may be constructed to behave electrically as any oneor more of a variety of such devices, and is not critical to the presentinvention. Thus, we only discuss the termination region hereafter.

[0020] Layers 403, 405 and 407 each include a charge profile whichdecreases as the layer is traversed laterally from region 421 to theoutside of the device. The charge profile is constructed, in thearrangement of FIG. 4, such that the total charge is a function oflateral or vertical position. Hence, as shown in FIG. 4, more charge isdeposited at 409 than at 411, and more charge is deposited at 411 thanat 413, and even less charge is deposited at 415. The charge profileincludes columns 409, 411, 413, and 415. The successive reduction oftotal charge with linear dependence on position results in an electricfield strength which is substantially constant over a lateral/verticalsection.

[0021] One way of accomplishing the decreasing charge profile uses thesame mask as used for fabrication of the active device. The mask has aportion that extends over the termination region. That extended portionhas plural openings which get smaller as one moves away from the activeregion. Thus, the discrete deposits of charge (i.e., charge dots)diminish in size, as the distance from the active region becomesgreater, resulting in a substantially constant electric field.

[0022] Additionally, a similar decrease in charge is encountered as avertical cross section is traversed. Specifically, looking only atcolumn 409 of FIG. 4, as one traverses upward from row 403 to 405 to407, less charge is deposited with each discrete deposit. The size ofthe openings in the mask used for each layer may vary, as depicted inFIG. 4.

[0023] The small rectangles represent a three by four matrix of discretedeposits of charge, each of which has a different volume of chargecontained therein. A typical geometry for such openings may range fromapproximately 2.5 to 40 microns squared.

[0024] The electric field strength at any position within thetermination region can be calculated from the spatial variation ofcharge. Alternatively, a desired electric field strength can be realizedby judicious design of the charge profile. The lateral charge at anypoint in the termination region can be obtained by summing thecontribution from each discrete charge region, subjected to a specifiedthermal anneal or drive. It is well known to those of skill in the arthow to calculate a charge profile for a desired field strength, and howto calculate the field strength from the desired charge profile.

[0025] Moreover, from FIG. 1 it can be seen that the approximately 600volts in the exemplary embodiment used herein must also be dropped fromthe bottom to the surface of the device. The same procedure is utilizedupwardly along any column, in order to drop 600 volts across N layers,for devices of N layers deep.

[0026] Notably, the invention may be fabricated in a convenient mannerfor multi layer devices. Specifically, in such multi layer devices thelayers are each fabricated separately using a particular mask. The samemasks can be utilized to lay down the termination region, with holes ofvarying size allowing for different amounts of charge. Notably, the maskfor each layer would be different, since the openings in the mask thatallow for the deposit of charge in the termination region are different.Thus, it is possible to have N different masks, one for each layer. Theportion of the mask that corresponds to the active region may be thesame for each mask, and the portion that corresponds to the terminationregion is different for each mask, in order to vary the profile.

[0027]FIG. 5 shows a slightly different embodiment for accomplishing asimilar objective as FIG. 4. Specifically, in FIG. 5, the size of thediscrete charge deposits is similar to each other. However, the distancebetween such discrete deposits varies as the termination region islaterally traversed. Moreover, the same varying spacing can be found asa vertical cross section is traversed. For example, distance 505 wouldbe slightly less than distance 506. Accordingly, the same substantiallyconstant electric field can be implemented by depositing the discretecharge deposits in equal amounts but further and further apart, as thecross section is traversed, rather than depositing the charge dots inless and less quantity at a fixed distance. Of course, a combinationapproach may be used as well, where the volume of charge in each depositand the spacing are varied.

[0028] In the preferred embodiment, the charge should be deposited in amanner that decreases with distance from the active (drift) region in asubstantially linear manner. This causes a substantially constantelectric field strength as one moves away from the active region. Thatmeans that the charge in each dot should decrease linearly with distancefrom the active region.

[0029] While the above describes a preferred embodiment of theinvention, various modifications and additions will be apparent to thoseof skill in the art.

What is claimed:
 1. A solid state device including a plurality oflayers, each layer including an active region and a termination region,at least two layers including termination regions that comprise avarying charge profile extending from the active region, laterallythrough the termination region towards the edge of said terminationregion.
 2. The device of claim 1 wherein all of said layers include avarying charge profile.
 3. The device of claim 1 wherein the varyingcharge profile of each layer is different from said varying chargeprofile for other layers.
 4. The device of claim 3 wherein said varyingcharge profile of each layer is different from said varying chargeprofile for other layers.
 5. The device of claim 4 wherein said varyingcharge profile on at least one layer comprises discrete deposits ofcharge that vary in total volume.
 6. The device of claim 4 wherein saidvarying charge on each layer comprises discrete deposits of charge thatvary in spacing from one another.
 7. The device of claim 4 wherein saidcharge profile on each layer varies substantially linearly with distanceaway from said active region.
 8. The device of claim 4 wherein chargevaries along a vertical cross section through multiple layers of saidtermination region at a fixed distance from said active region in asubstantially linear manner.
 9. The device of claim 7 wherein chargevaries along a vertical cross section through multiple layers of saidtermination region at a fixed distance from said active region in asubstantially linear manner.
 10. A solid state device having pluralepitaxial layers, each epitaxial layer including a termination region,the termination region being doped with a plurality of p- dots of chargewhich vary along the termination region in their volume or spacing, thevolume and spacing of said charge dots being different on at least twodifferent layers of said device.
 11. The solid state device of claim 10wherein the volume and spacing of the dots on each layer is such that asubstantially constant field strength is achieved moving away from theactive region along any layer or moving upwards through the layers alongany vertical cross section.
 12. A method of constructing a solid statedevice comprising the steps of: forming a first layer including anactive region and a termination region using a first mask; and formingat least a second layer including an active region and a terminationregion using a second mask, the first and second masks being differentfor the portions corresponding to the termination region.
 13. The methodof claim 12 wherein the step of forming at least a second layer includesusing a second mask that is substantially identical to the first mask inthe portion corresponding to the active region.
 14. A method of formingtermination region for a solid state device, the termination regionhaving a width and a depth, the method comprising the steps of: (a)doping the termination in varying charge concentrations along the width;and (b) doping the termination in varying charge concentrations alongthe depth.
 15. The method of claim 14 wherein the step (a) of dopingincludes placing discrete deposits of charge of varying volume along ahorizontal cross section of said termination region.
 16. The method ofclaim 14 wherein said steps (a) and (b) comprise doping inconcentrations such that field strength along any horizontal or verticalcross section is no greater than 15 volts per micrometer.